Despite the aforementioned visual strain caused by pseudoscopic perception, it is not always obvious to the viewer that he is positioned at a pseudoscopic viewing position.
U.S. Pat. No. 6,055,013 discloses an autostereoscopic display providing four views in each lobe, i.e., in each viewing cone. It is said that an observer has a wide range of viewing freedom with pseudoscopic viewing positions being present at the boundaries between adjacent lobes. A technique is illustrated for providing an observer with a visual indication of the pseudoscopic positions. For that purpose, a parallax optic is arranged for providing an indication in a central lobe that is dark whereas the indication in adjacent lobes is bright. The next lobes are then again dark, and so on. Thus, at pseudoscopic viewing positions, at least one eye of the observer sees a non-black position indication.
The technique of U.S. Pat. No. 6,055,013 thus allows an observer to distinguish between desired stereoscopic viewing positions and undesirable pseudoscopic viewing positions.